1. Field of the Invention
This invention relates to hydrogen absorbing alloy compacts for use as the negative electrodes of alkaline rechargeable (secondary) batteries, and a method of making the same.
2. Description of the Related Art
Hydrogen absorbing alloys are intermetallic compounds capable of absorbing and desorbing hydrogen electrochemically, and are chiefly used as electrode materials for the negative electrodes of alkaline rechargeable batteries. Such an alloy is hydrided during the charging of the electrode and dehydrided during the discharging thereof, so that it undergoes volume changes during charging and discharging. Consequently, it has been impossible to use such an alloy alone as an electrode, because alloy disintegration causes a failure of the electrode. For this reason, it has been common practice to use an electrode made by reducing a hydrogen absorbing alloy ingot to a powder of a certain particle size by mechanical grinding or the like, and then applying the powder to a current-collecting support such as foamed nickel or punching metal.
The structure of such a conventional electrode is shown in FIG. 2. In this conventional electrode, a binder 3 is used to prevent a hydrogen absorbing alloy powder 4 from falling off a current-collecting support 1. Moreover, an electrically conductive material 2 such as carbon or powdered nickel is added thereto for the purpose of securing good electrical conductivity between the hydrogen absorbing alloy powder 4 and the current-collecting support 1.
This conventional electrode is made by kneading a hydrogen absorbing alloy powder thoroughly with a binder and an electrically conductive material in an aqueous medium to form a slurry, applying this slurry to a current-collecting support, and pressing the resulting assembly.
Meanwhile, in order to enhance the electrode capacity of a negative electrode, it is effective to increase the proportion of the hydrogen absorbing alloy to the entire electrode. The best method is to make an electrode of a hydrogen absorbing alloy alone and thereby construct a nickel-hydrogen rechargeable (secondary) battery having a high capacity. According to this method, the present inventors made a porous body by forming a compact from a hydrogen absorbing alloy powder having a specific particle diameter (i.e., such a particle diameter as not to promote particle size reduction due to hydriding and dehydriding of the hydrogen absorbing alloy) without using a current-collecting support or a binder, and then sintering this compact. It has been reported that, since this porous sintered body does not contain any material incapable of contributing to the absorption and desorption of hydrogen (e.g., a current-collecting support or a binder), it has a greater amount of alloy loaded and hence a higher electrode capacity density. However, when such a sintered body electrode is made, it undergoes dimensional changes during sintering. Consequently, electrodes and like products requiring high dimensional accuracy must be machined, so that the amount of alloy corresponding to a margin for machining is wasted. Accordingly, it is desired to use an electrode comprising a compact in which no dimensional change occurs and, therefore, no alloy is wasted by machining.
In the case of a compact suffering no dimensional changes, it is also desired to make a desired electrode with high dimensional accuracy. Accordingly, it is intended to form a compact from an alloy powder alone. According to this method, the resulting compact scarcely needs to be machined and hence causes no appreciable alloy loss. However, when a compact formed simply by pressing an alloy powder is used as an electrode, the bulk density of the compact is lower than that of a sintered body, and the electrode capacity density of the electrode is reduced correspondingly. As used herein, the term xe2x80x9cbulk densityxe2x80x9d means the volume density of a sintered body or compact including its void space. Nevertheless, there are known several means for enhancing the electrode capacity density of the compact. They include, for example, using an alloy having a higher alloy capacity (mAh/g) per unit weight, and increasing the packing density by modifying the particle size distribution or by raising the compacting pressure. Moreover, a compact containing no binder has a low mechanical strength and, therefore, cannot withstand even the minimum required machining (e.g., machining for fine adjustment in the direction of the height). Furthermore, when such a compact is used as an electrode, the electrode is disintegrated as a result of charging and discharging, and the alloy powder constituting the compact electrode falls off. Thus, the electrode performs its function no more.
In the case of pressed compacts comprising a hydrogen absorbing alloy powder and a binder, various investigations on compacts for use in the storage of hydrogen and for use as the negative electrodes of alkaline rechargeable batteries have been made, as described in Japanese Patent Provisional Publication Nos. 1-119501, 8-7891, 9-31502, 4-181655 and 59-147032. However, in these prior art, the disintegration of the compact due to a fracture of the hydrogen absorbing alloy during the absorption and desorption of hydrogen is avoided by using a hydrogen absorbing alloy powder having a large particle diameter of several tens of micrometers for the formation of a compact, or by increasing the amount of binder contained in the compact. Nevertheless, when such a compact is used as the negative electrode of an alkaline rechargeable battery, a large amount of binder interferes with contact between hydrogen absorbing alloy particles and thereby increases the electric resistance. Moreover, the amount of hydrogen absorbing alloy present in the compact is decreased to cause a reduction in electric capacity when the compact is used as an electrode.
In view of the above-described problems of the prior art, the present inventors made intensive investigations with a view to developing a hydrogen absorbing alloy compact for use as the negative electrode of an alkaline rechargeable battery which, even when the compact containing no current-collecting support (or electrically conductive core) is used as an electrode, can prevent the compact electrode from being disintegrated by charge-discharge cycles and can achieve a desired discharge capacity, and which shows a further improvement in discharge characteristics by properly choosing the time of addition of an electrically conductive material. Also, it is another object of the present invention to provide a compact that is easily recycled.
As a result, the present inventor have now found that these problems can be solved by forming a porous compact having a specific bulk density from a hydrogen absorbing alloy powder alone and allowing a binder to be dispersed in the interstices formed by mutual contact of particles of the aforesaid hydrogen absorbing alloy powder and on the alloy surfaces. The bulk density (apparent density) represents a mass for the unit volume based on the geometric volume of outer shape of a material. The bulk density can be determined, for example, by mercury displacement or by aerometers.
In order to accomplish the above object, the present invention provides a hydrogen absorbing alloy compact for use as the negative electrode of an alkaline rechargeable battery which is made without the aid of a current-collecting support, contains a binder dispersed in the interstices formed by mutual contact of hydrogen absorbing alloy particles and on the alloy surfaces, and has a bulk density of 3.5 to 6.5 g/cm3.
Moreover, in the present invention, it is preferable that the aforesaid hydrogen absorbing alloy compact have a porosity of 15 to 55%. As used herein,.the term xe2x80x9cporosityxe2x80x9d means the proportion of the void space of the compact as based on the true density of the alloy [i.e., {1xe2x88x92(bulk density of compact)/(true density of alloy)}xc3x97100]. The true density is a density measured by immersing an alloy mass into water.
It is also preferable to use a water-soluble polymer as the aforesaid binder. The aforesaid binder is preferably contained in an amount of 0.01 to 2.0% by weight based on the aforesaid compact.
It is also preferable that the aforesaid hydrogen absorbing alloy powder have an average particle diameter of not greater than 20 xcexcm and an oxygen concentration of not greater than 0.50% by weight.
Moreover, in a preferred embodiment of the present invention, the hydrogen absorbing alloy powder contains an electrically conductive material mixed therewith during its grinding in an inert gas.
According to another aspect of the present invention, there is provided a method of making a hydrogen absorbing alloy compact for use as the negative electrode of an alkaline rechargeable battery which comprises the steps of forming a porous compact from a hydrogen absorbing alloy powder alone or a powder mixture of a hydrogen absorbing alloy powder and an electrically conductive material, without using a current-collecting support or a binder; impregnating the compact with an aqueous solution or solvent solution of a binder; and evaporating only the water or solvent contained in the compact as a result of the impregnation.
Thus, the present invention can provide a hydrogen absorbing alloy compact for use as the negative electrode of an alkaline rechargeable battery which, even when the compact containing no current-collecting support is used as an electrode, can prevent the compact electrode from being disintegrated by charge-discharge cycles owing to its high mechanical strength, and can achieve a desired discharge capacity, and which further exhibits an excellent high rate discharge property.
Another problem to be solved by the present invention is to provide a compact (in particular, a polyhedral compact) containing no current-collecting support (or electrically conductive core) but suitable for use as an electrode in which the strength of the compact is increased and its electrical conductivity is maintained, so that the compact electrode can be prevented from being disintegrated by charge-discharge cycles and a desired discharge capacity can be achieved.
As a result of intensive investigations made with a view to solving the above-described problem, the present invention provides a compact electrode which is made by forming a porous hydrogen absorbing alloy compact containing neither current-collecting support nor binder for the compact, impregnating the compact with an aqueous solution (or solvent solution) of a binder, and then drying the compact to remove the water (or solvent) alone, so that the strength of the compact is increased and good electrical contact between alloy particles is established. Moreover, the present invention also provides an alkaline rechargeable battery using this compact electrode.
The porous hydrogen absorbing alloy compact electrode having been impregnated with an aqueous solution of a binder in accordance with the present invention can provide an electrode having a high mechanical strength and suffering no specimen disintegration due to charge-discharge cycles.